San Diego: The biggest frustration for Phil Mickelson at this week's Farmers Insurance Open was his inability to transfer good practice form to the golf course rather than his surprising failure to make the cut.

The four-times major champion exited the tournament prematurely on Friday after shooting a four-under-par 68 in the second round, though the damage had already been done by his opening 77.

"It's easy for me to see where the difference is, and the difference is that I'm not taking it from the practice tee to the course," Mickelson told reporters after posting a one-over total of 145.

"My practice sessions for the last month or so have been better than they've ever been. I'm just having trouble bringing it from the practice session onto the golf course.

"I'm not sure exactly why that is, but the good news is that there's not an area of my game that I feel is bad."

Mickelson, a three-times winner at Torrey Pines, knew he needed to post a score of at least seven under on Friday to give himself a chance of making the cut.

"You can't go out expecting to shoot that," he said after mixing five birdies with a lone bogey on the easier North Course, one of two layouts hosting this week's event. "You have to let that round happen.

"You have to make a few birdies here and there, and capitalise on the par-fives. I didn't quite do enough to make it happen or let it happen. It was an okay round, but I needed to go pretty low." Practice Form

Asked when he was last unable to take good practice form to the course, Mickelson replied: "It happens, but it's unusual actually.

"When you feel good on the range, it's much easier to take it to the course. I'm not sure. It's been a little while, but it does happen occasionally."

Mickelson, who this week is competing in his second PGA Tour event of the year, was dumbfounded by his opening 77 on Thursday.

"Yesterday's round was shocking to me just because my expectations were so high because I knew I was playing well," the 41-year-old Californian said.

"To come out with that score was ... I don't know what else to say about it. I've got to let it go and move on."

Mickelson has had more than just golf on his mind at Torrey Pines this week with his 10-year-old daughter, Sophia, having suffered a small seizure at school last Friday.

She spent the weekend in hospital and underwent a series of tests on Thursday before returning to school on Friday.

"It's been a rough week but you get through it," Mickelson's wife, Amy, told the San Diego Union-Tribune. "She's held up pretty well. She's doing OK."